Brussels: NATO remains "cautiously optimistic" about progress in the war in Afghanistan despite the tactical challenges it has faced in recent months, the alliance's top military officer said on Wednesday.

Danish Gen Knud Bartels said Afghan security forces were becoming increasingly more effective in their fight against insurgents.

"Despite a number of tactical challenges in recent times, we remain cautiously optimistic that our plan is on track," said Bartels, who heads NATO's Military Committee, the alliance's highest military body.

He did not elaborate on the challenges, but said that this month alone nearly 100 Afghan soldiers and policemen have been killed in action. Thirty NATO service members also have died so far in April, bringing the total for this year to 122.

The general's caution is a more sobering take than NATO's upbeat official pronouncements on the 10-year war against the Taliban.

Since the beginning of the year, insurgents have launched a series of coordinated strikes in Kabul and several eastern cities, underscoring their ability to penetrate heavily guarded areas of the capital.

Relations between NATO and the Afghan government also have been strained by an Internet video of US Marines urinating on the corpses of presumed Taliban fighters, byQuran burning at a US base that sparked days of deadly protests and by the alleged killing spree of Afghan civilians by a US soldier.

Coalition forces, whose numbers reached a peak of over 140,000 troops last year, have already started a draw down. The vast majority are scheduled to leave by the end of 2014, when the Afghan troops are expected to take over all security responsibilities.

The US, which had about 100,000 service members in Afghanistan, expects to have a third of its troops out of the country by September.

"What is important now is that we must stay the course in accordance with (the alliance's) strategy," Bartels said.

Meanwhile, the Afghan army and police are being expanded rapidly, and are expected to grow to 352,000 by the middle of this year.

Bartels is chairing the meeting of the chiefs of staff of all 28 allied nations. US Gen Martin Dempsey, chairman of the Joint Chiefs of Staff, and Gen John Allen, top commander of US and international forces in Afghanistan, will brief participants.